Twitching Servos

[chris_t]

My Blade 400 has recently become un-flyable. All seems well during ground checks. All servos are working correctly, not sticking, no teeth missing.

Beginning to increase the throttle, all seems well until the 400 is about to take off, then the rotor head suddenly inclines from side to side violently and randomly.

I have identified the cause as one of the elevator servos. All other servos are fine. I swapped the elveator servo it out, and still have the same problem. So I looked at various forums and found two main possible reasons :

1) Spektrum DX6i Recall Consumer Notice | Horizon Hobby - Radio Control R/C Planes, RC Airplanes, RC Helis and Helicopters, RC Cars, RC Trucks, RC Truggys, RC Boats, RC Radios, RC Engines, parts, hop-ups, and accessories

This covers a re-call for stick potentiometers

2) Expo settings on the DX6i. I have no idea what the cause is, but several people have investigated and can repeat the problem.

One person did this :

Was running 80% on d/r and 20% on expo, Changed the d/r to 90% and the twitching stoped, but moved them back to 80% and it came back on every servo set to 80%.

Another did this : With no expo used on any control there is no problems. I can rule out the pot recall issue.

When I add + expo to the certain controls the twitches appear.

One example was, if I had:
=>1% expo on Aileron and,
>9% expo on Elevator and,
>9% expo on Rudder

I would get a twitch on the "pitch" servo (ch6) at a particular stick location. However if I drop any of these expos by 1% (even the rudder expo!!) the twitch would stop??

There are many other combos that cause one of the three servos to twitch at different locations. I have now found that if I switch between throttle hold and normal, with the same pitch curve on both. There are no twitches on throttle hold, regardless of expo settings!!
So I tried dropping the throttle curve to 0% accross the range in normal and checked for twitches. None!

And so : I've had a go at this and changed from the Blade 400 standard settings of 80% D/R and 20% Expo to 100% D/R and Expo Inhibit. Problem still there.

I'm getting a bit desperate now. Does anyone have any suggestions ?

Many thanks,

[Skipper Billy]

I think that as you can exhibit some control over the issue by changing things in the tx leads me to think that the issue is with the tx. Personally I would send it off for service - Horizon are pretty quick.

[cjcj1949]

I had problems like that with similar servos. The cause was a loose connection inside one servo. Servos tend to need replacing eventually anyway, so I would replace all cyclic servos and see if that cures the problem. I had a problem that the connection became loose just above take off revs. The servo then moved on its own and I had blade strikes. You could remove the blades and see if you lose control of one servo. If you do you could just replace that one. If it is a vibration issue it could be a rx or esc issue. Borrowing another tx might help to eliminate the tx. Blade 400 servos used to be unreliable, they may have improved. Servos do have a tough life on a 450.

[chris_t]

Good advice, thank you. My transmitter does not have a date code so it doesn't qualify for sending back under the DX6i recall.

I tried another DX6i with the same settings and got the same problem.

Although I have already replaced the servo, I suppose that might be faulty too, so I'm going to switch the left and right cyclic servos around and see where the fault goes.

[cjcj1949]

You could take the servos apart and check for loose wires. The wires should be supported with silicone holding them to the board, but the assemblers often miss. There is then a problem where the still flexible part of the wire meets the solder that wicks up the wire. That often breaks. There is a connection to the case of the motor as well and this can be a problem.

[chris_t]

Can't see a problem with loose wires on the servos fitted, but I also checked one I took out before and that had the problem you described, thank you.

I've borrowed another Blade 400 and swapped the gyro over. This made no difference. When I put my old one back on the problem switched from the cyclic to the elevator ! So I decided to swap over the receiver. This has cured the problem !

I'm now about to take the receiver apart to see if I can see anything obvious.

[cjcj1949]

Good stuff. Could be a dodgy pin on the connector. Dirty or bent. Swapping is the best way, but you might still be wrong. You need to swop a few times as sometimes faults disappear after swapping. It is very easy to believe what you want to believe.

[chris_t]

OK, I looked for a dodgy pin, and couldn't find anything, but re-flowed the solder joints on the connector on the receiver. Re-fitted and all seemed fine. Now it gets really peculiar. On the next flight the receiver binding went fine, the red light came on, and the twin tone sounded, but the red light on the gyro continued flashing. I had all controls as normal except rudder.

To cut a long story short, I realised that each time I took it apart it worked fine, and each time I put it back together again the gyro light flashed. The cover on the receiver is only a piece of plastic card folder over and stuck with double sided tape, but it has to protect the sensitive receiver from interference so I wondered if it was conductive and was partially shorting out the connector pins. Sure enough, my multi-meter read about 100 K ohms, so I wrapped a load of cling film round the circuit board and put it back inside the cover. Since then it flies perfectly.

How weird is that ?

[aparkin]

which rx is m8 i have a little micro specky reciver doing strange things 2 if its the same1 i may shrink rap it and try it out again

[cjcj1949]

It is very easy to believe what you want to believe.

You've gone one better and not only thought outside the box, but realised that the box was the problem. Conductive boxes do usually get earthed or connected to a suitable reference point like 0v. The motor cans in the servos are usually earthed for want of a better term. Brushless motors aren't. Glad you've found a solution, might be worth emailing horizon and see what they say. You might get a new cover and a connection point.

[chris_t]

Good point. Horizon have been very helpful before, I'll see what they say. Thread closed !